Fluid-structure Problems Research Articles

Study of a complex fluid-structure dam-breaking benchmark problem using a multi-phase SPH method with APR

The present work is dedicated to an accurate modeling of violent Fluid-Structure-Interaction (FSI) problems using a coupled Lagrangian particle method combining a multi-phase δ-SPH scheme and a Total-Lagrangian-Particle (TLP) method. Advanced numerical techniques, e.g. Adaptive-Particle-Refinement (APR), have been included in the particle method for improving the local accuracy and the overall numerical efficiency. On one hand, this paper aims to demonstrate the capability of the proposed numerical method in modeling FSI flows with large density-ratios, strong fluid impacts, complex interfacial evolutions and considerable wall-boundary movements and deformations; On the other hand, the numerical results presented in this paper show the importance of considering the existence of air-phase in some complex FSI problems. The entrapped air-bubble, after the free-surface rolling and closing, plays an important role in the overall flow evolution and hence the hydrodynamic load on the structure. Although a density ratio as large as 1000 has been adopted, clear and sharp multi-phase interfaces, which undergo violent breakups and reconnections, are present in the numerical results, and more importantly, stable and smooth pressure fields are obtained. This contributes to an accurate prediction of the structural response, as validated by both the experimental data and other numerical results.

Theoretical formulation and seamless discrete approximation for localized failure of saturated poro-plastic structure interacting with reservoir

This paper deals with nonlinear fluid-structure problems brought about by progressive localized failure of a dam structure built of porous cohesive material in interaction with reservoir under extreme static and/or dynamic loads. The theoretical formulation for structure is based upon Biot’s porous media theory extended to localized poro-plasticity that provides a sharp representation of cracks saturated with fluid. The fluid-structure interaction is handled by a seamless discretization between structure and fluid achieved by using a judicious combination of Voronoi cell approximation for structure, finite element approximation for fluid saturating cracks and finite element approximation for outside fluid. This is achieved by exploiting the duality of Voronoi cell and Delaunay triangle representations to allow exchanging information between mechanics and pore pressure fields at the numerical integration points to account for internal fluid-structure interaction, as well as with the external fluid motion in the reservoir being limited to small (irrotational) motion, described by Lagrangian description and mixed discrete approximation. Numerical simulations illustrate an excellent performance of the proposed model, capable to provide the overall safety assessment for pore-saturated structures, with outside fluid acting as the source of pore saturation and the external loading, in both quasi-static and dynamic setting.

Measurement and analysis of sound radiation from coherently vibrating shunt reactors

In parallel with the rapid development of ultra-high voltage (UHV) power transmission systems, noise contamination radiated from UHV shunt reactors is becoming an urgent concern. This investigation explored the distributed structural vibration of reactors and their near-field and far-field sound radiation characteristics based on a thorough on-site measurement and a combined numerical method. The proposed method could realize sound field reconstruction of on-site shunt reactors by directly measuring their surface vibration, thus avoiding complex modelling with fluid-structure interactions and multi-physics problems. Furthermore, the combined method was utilized to investigate the effect of acoustical environments upon reactors’ sound radiation. The results show that reflection boundaries composed of firewalls among reactor units could introduce a higher noise level in the parallel radiation region due to their influence on the sound source directivity. Finally, the sound power levels of the on-site reactors were derived from the constructed sound field, which could avoid on-site background noise concerns in the current executive standards.

Study of modal analysis based on fluid-structure interaction

Abstract In this work, a coupled fluid-structure problem is approached, comparing the result with the modal analysis of a structure. The objective of this work is to analyze the physical phenomenon of fluid-structure interaction of a flexible structure. For this, the coupled problem solved using an Arbitrary Lagrangean-Eulerian (ALE) approach. As support for solving the mathematical equations of coupled problem, ANSYS® physical analysis software was used. An experimental modal analysis, using the Rational Fractional Polynomial method was developed for a small scale steel structure, and the result of this was compared with the result obtained from the model simulated in the software. Their vibration modes and natural frequencies obtained by numerical modeling were validated experimentally. Whit the numerical modeling of the modal analysis of a structure experimentally validated, attempted to analyze the dynamic behavior of the structure when it is subjected to a load due to a fluid-flow through a coupled fluid-structure problem. The results presented in this work show that the structure subjected to loads due to the fluid-flow, moves according to its vibration modes.

Analysis and assessment of a monolithic FSI finite element method

In this paper we analyze and investigate numerically a monolithic finite element method for the incompressible Navier–Stokes equations coupled to the Saint Venant–Kirchhoff hyperelastic model. The approach strongly enforces the coupling conditions on the fluid–structure interface and treats both solid and fluid equations in a reference domain accounting for geometric motion through time-dependent coefficients. The paper derives an energy balance for the fully discrete system and proves that the finite element method is numerically stable. The performance of the method is further assessed by validating numerical results for the pressure impulse prorogation test and against the results of a recently proposed experimental 3D fluid-structure interaction benchmark problem.

Shock response of electrostatically coupled microbeams under the squeeze-film damping effect

This paper presents a theoretical investigation of the dynamic response of electrostatically coupled microcantilever beams under the combined effect of squeeze-film damping and mechanical shock for MEMS applications. We consider two different microsystem designs, namely single and dual beams, operating at varying conditions to explore their possible use based on the application of interest. The single-beam system is actuated via a fixed electrode (uncoupled actuation), while the electric actuation of the dual-beam system, comprising two movable microbeams, is achieved by applying a DC voltage among them (coupled actuation). We develop a multi-physics model to simulate the dynamic response of single and dual microbeams while accounting for the fringing field effect, the squeeze-film damping resulting from the interactions between the vibrating microbeams and the surrounding fluid, and the mechanical shock. The squeeze-film damping is incorporated using a nonlinear analytical expression rather than solving the fully coupled fluid–structure problem. Numerous studies have shown the validity of this analytical approach under the assumption of long beams and neglecting the fluid compressibility. Some benchmark cases are considered to verify the predictive capability of the numerical model. The simulation results are in good qualitative and quantitative agreements with those reported in the literature. A parametric study is then conducted to investigate the effects of the initial gap distance, the fluid viscosity, and the beam geometry on the shock response of the microsystem. The squeeze-film damping enables the protection of the vibrating microbeams from hitting the substrate under mechanical shock and can be exploited to enhance the reliability of MEMS devices. However, this protection can be significantly reduced by increasing the gap distance or decreasing the operating pressure. The numerical study shows that the dual-beam systems withstand better to a mechanical shock. Breaking the symmetry of the dual-beam system in terms of the beams’ geometry is found to reduce significantly the resistance to shocks. On the other hand, given their high sensitivity to a mechanical shock, single-beam systems are observed to be more attractive for deployment as microswitches.

Direct simulation of rigid particles in a viscoelastic fluid

In this paper, we present a numerical method which performs the direct simulation of 2D viscoelastic suspensions. Objectives. Interactions between viscoelastic fluids of Oldroyd type, including inertial effects, and a large number of rigid disks or ellipsoids are addressed. Method. The numerical method is built upon a fictitious domain approach which consists in defining the fluid-structure problem over the whole domain (fluid+solid), see e.g. [1]. In the Newtonian case [2, 3], the resulting variational formulation, although classical, uses non-standard functional spaces which include the rigidity constraints. From the numerical point of view, these constraints can be addressed by penalty methods, leading to the possible use of standard finite elements solvers with fixed structured meshes. We show how this approach can be adapted to viscoelastic fluids of Oldroyd type. Results. Two types of configurations are investigated in dilute and dense regimes: sedimentation of circular particles and shear flows with ellipsoidal particles.

Experimental investigation of flow field around the elastic flag flapping in periodic state

The flapping of a flag in the wind is a classical fluid-structure problem that concerns the interaction of elastic bodies with ambient fluid. We focus on the desirable experimental results of the flow around the flapping flag. By immersing the elastic yet self-supporting heavy flag into water flow, we use particle image velocimetry (PIV) techniques to obtain the whole flow field around the midspan of the flag interacting with a fluid in periodic state. A unique PIV image processing method is used to measure near-wall flow velocities around a moving elastic flag. There exists a thin flow circulation region on the suction side of the flag in periodic state. This observation suggests that viscous flow models may be needed to improve the theoretical predictions of the flapping flag in periodic state, especially in a large amplitude.

A variationally bounded scheme for delayed detached eddy simulation: Application to vortex-induced vibration of offshore riser

We present a bounded and positivity preserving variational (PPV) method for the turbulence transport equation of Spalart–Allmaras based delayed detached eddy simulation (DDES). We employ the developed solver to simulate the vortex-induced vibration of a slender flexible riser immersed in a turbulent flow. The fluid-structure interface problem is solved by recently proposed partitioned iterative scheme [1], which consists of nonlinear force correction for a stable coupling of the Navier–Stokes equations with the low-mass structure subjected to strong inertial effects from the surrounding incompressible flow. In our fluid-structure model, we consider the flexible cylindrical riser as a long tensioned beam via linear modal analysis. In the present contribution, we first validate the PPV-based DDES solver for the flow past a three-dimensional stationary cylinder at two subcritical Reynolds numbers of Re=3900 and Re=140,000 based on the diameter of the cylinder. We next simulate a three-dimensional flexible riser model under a pinned-pinned condition at Re=4000 and compare our results with that of the experimental measurements. We assess the response characteristics at various locations along the span of the riser and discuss the orbital trajectories at those locations. In particular, the numerical study emphasizes on (i) The effectiveness of the modal analysis in modeling the riser dynamics and a positivity-preserving variational method in DDES for turbulence modeling, (ii) validation of response amplitudes and motion trajectories, and (iii) new insights on the trajectories and vortical structures of the flexible riser undergoing vortex-induced vibrations (VIV). We confirm a standing wave response and complex chaotic response of riser VIV observed in the recent experiments.

Isogeometric analysis for acoustic fluid-structure interaction problems

Modeling and analysis of the dynamic behavior of acoustic fluid-structure interaction problems are important in understanding engineering structural systems. Characterization of such coupled systems through analytical modeling is limited and often numerical modeling is sought-after. Recently, Isogeometric Analysis (IGA) has gained prominence in modeling and analyzing complex systems. In the present study, IGA with Non-Uniform Rational B-Splines (NURBS) is used to characterize the acoustic fluid-structure interaction problems and the performance is compared with the classical Finite Element solution. The gain in computational performance of IGA is demonstrated with case studies using a beam and a plate coupled to a cavity filled with an acoustic medium. To illustrate further, an application of acoustic fluid-structure problem using IGA, a car cavity in two dimension coupled to a beam is also demonstrated.

Realization of hydrodynamic experiments on quasi-2D liquid crystal films in microgravity

Freely suspended films of smectic liquid crystals are unique examples of quasi two-dimensional fluids. Mechanically stable and with quantized thickness of the order of only a few molecular layers, smectic films are ideal systems for studying fundamental fluid physics, such as collective molecular ordering, defect and fluctuation phenomena, hydrodynamics, and nonequilibrium behavior in two dimensions (2D), including serving as models of complex biological membranes. Smectic films can be drawn across openings in planar supports resulting in thin, meniscus-bounded membranes, and can also be prepared as bubbles, either supported on an inflation tube or floating freely. The quantized layering renders smectic films uniquely useful in 2D fluid physics. The OASIS team has pursued a variety of ground-based and microgravity applications of thin liquid crystal films to fluid structure and hydrodynamic problems in 2D and quasi-2D systems. Parabolic flights and sounding rocket experiments were carried out in order to explore the shape evolution of free floating smectic bubbles, and to probe Marangoni effects in flat films. The dynamics of emulsions of smectic islands (thicker regions on thin background films) and of microdroplet inclusions in spherical films, as well as thermocapillary effects, were studied over extended periods within the OASIS (Observation and Analysis of Smectic Islands in Space) project on the International Space Station. We summarize the technical details of the OASIS hardware and give preliminary examples of key observations.

Adaptive unified continuum FEM modeling of a 3D FSI benchmark problem.

In this paper, we address a 3D fluid-structure interaction benchmark problem that represents important characteristics of biomedical modeling. We present a goal-oriented adaptive finite element methodology for incompressible fluid-structure interaction based on a streamline diffusion-type stabilization of the balance equations for mass and momentum for the entire continuum in the domain, which is implemented in the Unicorn/FEniCS software framework. A phase marker function and its corresponding transport equation are introduced to select the constitutive law, where the mesh tracks the discontinuous fluid-structure interface. This results in a unified simulation method for fluids and structures. We present detailed results for the benchmark problem compared with experiments, together with a mesh convergence study.

Numerical approximations of flow induced vibrations of vocal folds

The paper focus on mathematical modelling of incompressible fluid flow interacting with vibrations of an elastic vocal fold. The flow in moving domain is modelled by the incompressible Navier-Stokes equations written in the Arbitrary Lagrangian-Eulerian (ALE) form. The channel geometry is an approximation of the human glottal region. The flow model is coupled with a simplified structure model. The problem is mathematically described and the resulting fluid-structure interaction problem is discretized by a stabilized finite element method. A strong coupling algorithm is applied for solution of the coupled fluid-structure problem. The choice of boundary conditions is discussed, particularly the choice of different artificial inlet/outlet boundary conditions is described in details. The numerical results are shown.

A level set-based topology optimization for fluid-structure coupled problems by using two-phase material model

In this study, we develop a two-phase material model mixed with a solid and fluid phases to represent an elastic material and an acoustic material in a unified set of governing equations. By applying the model, boundary conditions at boundaries between different two-phase materials are satisfied naturally, and the boundaries do not have to be expressed explicitly. This characteristic is effective for topology optimization of fluid-structure coupled problems by using finite element method. Consequently, we formulate an optimization method utilizing the two-phase material model and a level set method to express boundaries between different two-phase materials. An approximate method to obtain topological derivative is also developed by applying the variational analysis and the adjoint variable method. Topological derivative of an objective functional is considered to be proportional to the derivative of the level set functions with respect to time and the values of the level set functions are updated by solving time evolutionary reaction diffusion equation with a regularization term. After the verifications of the two-phase model, we present several numerical demonstrations for directional acoustic cloaking of a cylindrical obstacle placed in unbounded air medium.

Large-eddy simulation, fuel rod vibration and grid-to-rod fretting in pressurized water reactors

Grid-to-rod fretting (GTRF) in pressurized water reactors is a flow-induced vibration phenomenon that results in wear and fretting of the cladding material on fuel rods. GTRF is responsible for over 70% of the fuel failures in pressurized water reactors in the United States. Predicting the GTRF wear and concomitant interval between failures is important because of the large costs associated with reactor shutdown and replacement of fuel rod assemblies. The GTRF-induced wear process involves turbulent flow, mechanical vibration, tribology, and time-varying irradiated material properties in complex fuel assembly geometries. This paper presents a new approach for predicting GTRF induced fuel rod wear that uses high-resolution implicit large-eddy simulation to drive nonlinear transient dynamics computations. The GTRF fluid–structure problem is separated into the simulation of the turbulent flow field in the complex-geometry fuel-rod bundles using implicit large-eddy simulation, the calculation of statistics of the resulting fluctuating structural forces, and the nonlinear transient dynamics analysis of the fuel rod. Ultimately, the methods developed here, can be used, in conjunction with operational management, to improve reactor core designs in which fuel rod failures are minimized or potentially eliminated. Robustness of the behavior of both the structural forces computed from the turbulent flow simulations and the results from the transient dynamics analyses highlight the progress made towards achieving a predictive simulation capability for the GTRF problem.

Arbitrary Lagrangian Eulerian Formulation for Sloshing Tank Analysis in Nuclear Engineering

Fluid-structure interaction plays an important role in nuclear engineering design, where several numerical and experimental tests need to be performed on new tank design before getting into the production process. The design can be performed for fluid storage tanks that require knowledge of sloshing frequencies and hydrodynamic pressure distribution on the structure. These can be very useful for engineers and designers to define appropriate material properties and shell thickness of the structure to be resistant under seismic loading. Data presented in current tank seismic design codes such as Eurocode are based on simplified assumptions for the geometry and material tank properties. Fuel tanks may undergo different types of loading, including seismic loading, where the behavior of storage tanks includes material nonlinearities, which are caused by material yielding. The Arbitrary Lagrangian Eulerian formulation based on finite element analysis presented in the paper takes into account material properties of the structure as well as the complex geometry of the tank. The formulation uses a moving mesh with a mesh velocity defined through the structure motion. In this paper, we use different approaches to solve a fluid-structure coupling problem. The first one uses the full Navier-Stokes equation for the fluid with projection method, and the second approach uses potential flow theory. The problem consists of a sloshing deformable tank submitted to acceleration loading.

Wave Response Analysis for Pontoon-type Pier: Very Large Floating Structure

In this study, we proposed a pier of pontoon-type, Very Large Floating Structure (VLFS), with the length of 500m, breadth of 200 m and height of 2 m in Yeosu domestic port. Since this structure ought to endure wave loads for long periods at sea, it is essential to analyze the wave response characteristics. Direct-method is used to analyze the fluid-structure problem and the coupled motion of equation is used to obtain response results. The structural part is calculated by using finite element method (FEM) and the fluid part is analyzed by using boundary element method (BEM). Dynamic responses caused by the elastic deformation and rigid motion of structure are analyzed by numerical calculation. To investigate response characteristics of the pier in regular waves, several factors such as the wavelength, water depth, wave direction and flexural rigidity of structure are considered. As a result, wave response of pier changed at the point of 1.5 and represented the torsional phenomenon according to the various incident waves. And the responses showed increasing tendency as the water depths increase at the incident point in case of $L/{\lambda}

Nonconforming mixed finite element approximation of a fluid-structure interaction spectral problem

We aim to provide a finite element analysis for the elastoacoustic vibration problem. We use a dual-mixed variational formulation for the elasticity system and combine the lowest order Lagrange finite element in the fluid domain with the reduced symmetry element known as PEERS and introduced for linear elasticity in [1]. We show that the resulting global nonconforming scheme provides a correct spectral approximation and we prove quasi-optimal error estimates. Finally, we confirm the asymptotic rates of convergence by numerical experiments.

Active Transient Sound Radiation Control from a Smart Piezocomposite Hollow Cylinder

Abstract The linear 3D piezoelasticity theory along with active damping control (ADC) strategy are applied for non-stationary vibroacoustic response suppression of a doubly fluid-loaded functionally graded piezolaminated (FGPM) composite hollow cylinder of infinite length under general time-varying excitations. The control gain parameters are identified and tuned using Genetic Algorithm (GA) with a multi-objective performance index that constrains the key elasto-acoustic system parameters and control voltage. The uncontrolled and controlled time response histories due to a pair of equal and opposite impulsive external point loads are calculated by means of Durbin’s numerical inverse Laplace transform algorithm. Numerical simulations demonstrate the superior (good) performance of the GA-optimized distributed active damping control system in effective attenuation of sound pressure transients radiated into the internal (external) acoustic space for two basic control configurations. Also, some interesting features of the transient fluid-structure interaction control problem are illustrated via proper 2D time domain images and animations of the 3D sound field. Limiting cases are considered and accuracy of the formulation is established with the aid of a commercial finite element package as well as comparisons with the current literature.

Role Of Computational Simulations In The Design Of Piston Rings

Abstract The paper presents computational approaches using modern strategies for a dynamic piston ring solution as a fluid structural problem. Computational model outputs can be used to understand design parameter influences on defined results of a primarily integral character. Piston ring dynamics incorporates mixed lubrication conditions, the influence of surface roughness on oil film lubrication, the influence of ring movement on gas dynamics, oil film formulation on a cylinder liner and other significant influences. The solution results are presented for several parameters of SI engine piston rings.